The invention relates to method for preventing stress corrosion cracking of bearings. It particularly relates to preventing stress corrosion cracking of bearing inner rings used in high speed cryogenic rotating machinery, such as turbopumps. Rocket engines or vehicles operating around space stations, orbital transport vehicles, and engines for high-orbit satellites may have turbopumps which run at very high speeds. The bearing material used in the Rockwell International Corp. designed Orbital Transfer Vehicle cryogenic engine turbopumps is AISI type 440C stainless steel which has a coefficient of thermal contraction less than the shaft material. This cryogenic engine's fuel turbopump runs at approximately 110,000 rpm. Due to this high speed and the differential expansion characteristic of the bearings and shaft, the bearing inner rings must be installed with relatively tight fits at room temperature (producing a hoop stress of about 50,000 psi). This tight fit is required in order to maintain a defined operational fit during cryogenic operation and to control the rotordynamic behavior of the turbomachine's rotor.
During bearing installation on the turbopump shafts, bearing inner rings are heated with a heating pad up to +300.degree. F. while the shafts are chilled in liquid nitrogen (-320.degree. F.). The bearing inner rings are quickly slipped over the shaft and the assembly is allowed to return to room temperature before continuing with the build. No other installation procedure has been developed for these tight interference fit bearing rings. This operation permits the accumulation of moisture beneath the bearing inner rings, which, if combined with the high hoop stress, can cause stress corrosion and resultant bearing inner ring cracking.
The rate at which stress corrosion cracking occurs is highly dependent on the magnitude of the hoop stress in the presence of moisture. Based on static test data developed for the Space Shuttle Program, the unprotected bearing inner rings of the Rockwell International designed MK49 turbopumps (50,000 psi hoop stress) were projected to crack within four days of installation--without ever having been subjected to operational spin testing.
Several methods were investigated for improving or limiting stress corrosion cracking behavior of AISI type 440C inner bearing rings. For example, by assembling a bearing inner ring on a shaft in as dry an environment as possible, and keeping the assembly dry, the time to failure is greatly extended. However, during pump assembly, storage and use it is extremely difficult to eliminate all the moisture from the system at all times. Therefore, this method by itself was determined to be impracticable. Nevertheless, a good practice is to attempt to keep the internal areas of the turbomachine reasonably dry.
Another method considered for improving or limiting stress corrosion cracking behavior was the possibility of changing the material surface composition to render it less reactive to the environment. For example, changing the surface chemistry by implantation of a corrosion resistant material such as chromium. However, this method involved a relatively high number of unknown factors and it was not known if the corrosion characteristics of 440C stainless steel could be altered sufficiently to significantly offset its stress corrosion cracking behavior. The method was therefore not tested.
The use of a less susceptible material than 440C was investigated. However, few materials perform satisfactorily as antifriction bearing materials under cryogenic conditions. Testing of these materials did not show any advantage with respect to stress corrosion cracking.
Redesigning the turbopump to reduce the stresses on the bearing inner rings was considered; however, redesign was not practical because of the relatively small size of the turbopump shafts and the limited space surrounding the bearings.